sustainability Article Fertilization Failed to Make Positive Effects on Torreya grandis in Severe N-Deposition Subtropics Yini Han 1,2, G. Geoff Wang 3, Tonggui Wu 4, Wenjing Chen 1,2, Yongliang Ji 1 and Songheng Jin 1,2,* 1 Jiyang College, Zhejiang A&F University, Zhuji 311800, China; [email protected] (Y.H.); [email protected] (W.C.); [email protected] (Y.J.) 2 State Key Laboratory of Subtropical Silviculture, School of Forestry and Biotechnology, Zhejiang A&F University, Hangzhou 311300, China 3 Department of Forestry and Environmental Conservation, Clemson University, Clemson, SC 29634, USA; [email protected] 4 East China Coastal Forest Ecosystem Long-Term Research Station, Research Institute of Subtropical Forestry, Chinese Academy of Forestry, Hangzhou 311400, China; [email protected] * Correspondence: [email protected]; Tel.: +86-575-87760007 Abstract: In managed orchards, fertilization brings out not only high productivity expectations but also severe environmental pollution. Because economic profit takes priority over environmental cost, increasing amounts of fertilizer have been used in mature subtropical Torreya grandis orchards. However, given the magnitude of global nitrogen deposition, it’s worth considering whether heavy fertilizer treatment is necessary. To elucidate the balance between T. grandis nutrient demands and fertilizer supply, we determined the C, N, and P concentrations of foliar and soil ([C], [N], [P]) at 9 orchards undergoing long-term fertilizer treatments in two scenarios of N and N + P addition with Citation: Han, Y.; Wang, G.G.; Wu, different intensity. After documenting the dynamic variation of plant growth, nutrients characteristic, T.; Chen, W.; Ji, Y.; Jin, S. Fertilization and the corresponding resorption efficiency, we found that excessive N addition interfered T. grandis’ Failed to Make Positive Effects on Torreya grandis in Severe sensibility to P availability in this N-enrichment area, leading to an increasing foliar [P] and resorption N-Deposition Subtropics. efficiency (PRE) and decoupling plant C:N:P ratios. As a result, enhanced fertilizer supply failed Sustainability 2021, 13, 9736. https:// to improve carbon accumulation, plant growth, and yield effectively. These results demonstrate doi.org/10.3390/su13179736 that extra fertilization in the N-saturated study area highly reduced the economic and ecological efficiency of fertilizers. Thus, our research suggests that N addition in the studied orchards should be Academic Editors: Ram rejected, and we recommend organic management as a more conducive method to achieve sustainable Swaroop Meena, Manoj development. Kumar Jhariya, Sandeep Kumar and Gulab Singh Yadav Keywords: Torreya grandis; nitrogen enrichment; long-term fertilizer treatments; phosphorus limitation Received: 1 August 2021 Accepted: 27 August 2021 Published: 30 August 2021 1. Introduction Torreya grandis T. grandis Publisher’s Note: MDPI stays neutral ( ) is an old relict species within the family of Taxaceae that with regard to jurisdictional claims in is endemic in China, often referred to as Chinese Torreya. It has been one of the most published maps and institutional affil- economically important tree species in the subtropical region of China. The Kuaijishan iations. Ancient Chinese Torreya Community was listed in China National Important Agricultural Heritage and Globally Important Agricultural Heritage Systems (GIAHS) in 2013 [1,2]. Trees in the community area are considered ‘living fossils’ because they originated from the application of grafting and artificial selection techniques in ancient China. Many of those trees are over one thousand years economic old but still sustain a high yield of seed Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. production. The Chinese Torreya community is highly valued both ecologically (e.g., water This article is an open access article and soil conservation, climate regulation, biodiversity maintaining) and economically (e.g., distributed under the terms and nuts, medicine, oil, and as an ornamental tree). As a result, the planting area of T. grandis conditions of the Creative Commons has steadily increased in several provinces during the past three decades. However, the old Attribution (CC BY) license (https:// Torreya plantation in the Kuaijishan Ancient Chinese Torreya Community is facing severe creativecommons.org/licenses/by/ degradation due to nature aging, climate change, and inappropriate management, such as 4.0/). overfertilization. Sustainability 2021, 13, 9736. https://doi.org/10.3390/su13179736 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 9736 2 of 14 Alternate bearing phenomenon loom large, due to the overexploitation of T. grandis seeds; to improve this, escalating amounts of chemical nitrogen (N) and phosphorus (P) rich fertilizer have been applied by the local manager, without any scientific management guidance (e.g., supplying based on the demands of plants). N and P are key nutrients that play pivotal roles in controlling plant growth and litter decomposition, as well as in ecosystems’ biochemical cycles [3–5]. However, the indiscriminate use of chemical fertilizer (including quantity and proportion) has caused abnormally high concentrations of N and P to accumulate in the soil, which has severely stressed the terrestrial plants’ multiple physiological processes. Furthermore, the excessive use of chemical fertilizer has also generated serious adverse environmental consequences, such as non-point source pollution [6] and N-induced soil acidification (especially due to NH4+), both of which have been observed in multiple ecosystems [7–9]. It should be noted that soil acidification would change ecosystem biogeochemistry, accelerating cationic nutrient (such as Ca2+, Mg2+) leaching and thereby, reducing plant productivity in turn [10,11] and negatively affecting local biodiversity [12,13]. Under natural conditions, N input to an N-limited ecosystem (such as boreal forests) will improve net primary productivity (NPP) through a direct fertilizing effect on vege- tation [14]. As a direct result from global change, N deposition in many regions of the world (i.e., United States, western Europe, and China) currently exceeds 10 kg N ha−1 yr−1, especially in tropical and subtropical areas of China [15,16],where over 80–120 kg N ha−1 yr−1 has been reported [17]. China has become one of the highest N deposition area, and N deposition has increased by approximately 60% over the past three decades [16]. A meta-analysis has reported an N saturation threshold of 50–60 kg ha−1 yr−1 across the entire terrestrial ecosystem [18]. Therefore, as N deposition levels continue to accelerate, N-limitation has been subsequently alleviated [19], and there is a high likelihood that the ecosystem is shifting to an N-enriched status [20]. Unlike N, which can be accumulated through biological fixation and increasing deposition, available soil P comes from slow parent mineral weathering and a low rate of atmospheric deposition from wildfires. Based on nutrient demand balance theory, excessive N input has been reported to not only disturb the balance in the biogeochemical cycles of essential plant nutrients [21–23], but also initiate P-limitation in forest ecosystems [24–27]. Fang et al. [28] reported N saturation in three subtropical sites and noted that P deficiency is becoming progressively more problematic. The anthropogenic alternation of regional P and N cycling has led to large areas of southern China forests shifting to human-induced P-limitation [27]. NPP/GPP has transformed from being N-limited to P-limited in many forest ecosystems [4,26,29,30]. Soil N and P availability, especially when combined with N deposition and/or external addition, can influence forest productivity and ecosystem processes [3,31]. Trees can keep leaf nutrient concentrations and their ratios stable by modulating the nutrients coming from branches, roots or senescent leaves [32,33]. Recently, foliar N and P concentration stoichiometry ratios (i.e., N:P, C:N, C:P) have been used to indicate soil N- and/or P- limitations on plant growth [24,34,35]. In addition, nutrient resorption [nitrogen (NRE) and/or phosphorus (PRE)] from senescing plant tissues and the proficiency of nutrient conservation [36], are also widely used as indicators in studies of nutrient cycling between plants and soil in fluctuating environments. Generally, nutrients would transfer from senescent leaf to trunk before falling off, thereby maintaining the plant nutrients at a favorable level. The NRE and PRE might depend on the type of nutrient limitation [37] and vary in response to the plant’s intrinsic genetic characteristics [38,39]. NRE/PRE is commonly employed to determine the relative limitation between N and P; NRE/PRE values > 1 imply a stronger N-limitation at the ecosystem scale [26]. However, previous studies predominantly focused on the effect of single N or P addition on leaf nutrients or resorption; thus, the effect of multiple nutrient addition on NRE and/or PRE in subtropical forests is not well understood. Meanwhile, the relationship between plant P uptake and nutrient environmental supply is more comprehensive than N. Under intensive management, the long-term addi- Sustainability 2021, 13, 9736 3 of 14 tion of balanced compound fertilizer may cause excess P in orchards, as plants generally require less P than N [40]. P is an essential element for nucleic acids and membrane lipids. Although P sensing and signaling are not fully understood,